1. Field of the Invention
The present invention relates to polyvinyl alcohol based fibers of strong resistance to fatigue, and also relates to dipped cords of strong resistance to fatigue that comprise the polyvinyl alcohol based fibers, and to a method for producing the dipped cords. The invention further relates to a rubber-reinforcing material of strong resistance to fatigue that comprises the dipped cords.
2. Discussion of the Background
Fibers have been widely used in reinforcing materials for cement, rubber, resin, etc. For rubber-reinforcing materials, for example, dipped cords are widely used that are prepared by specifically processing fiber bundles for enhancing their adhesiveness to rubber, for example, through RFL (resorcinol-formalin latex) treatment. Such dipped cords are required to have not only good mechanical properties but also other properties such as fatigue resistance, etc. In particular, where dipped cords are used for tire cords, brake hose-reinforcing materials, belt reinforcing materials and others, they are required to have high-level properties of pressure resistance, fatigue resistance, dimensional stability, etc.
For rubber-reinforcing materials, polyvinyl alcohol (PVA) based fibers, rayon fibers, polyester fibers, nylon fibers, aramide fibers, etc., have heretofore been generally used. However, these fibers have some problems. The strength of rayon fibers is low. Using large quantities of such weak rayon fibers is inevitable for ensuring elevated reinforcement, which, however, causes production cost increase and up-sizing of products. Nylon fibers are greatly deformed by external stress applied thereto, as having a low modulus of elasticity. Polyester fibers are strong and have a high modulus of elasticity in some degree, but their adhesiveness to rubber is poor. In addition, since they often shrink after being vulcanized, the dimension of products containing them will vary. Aramide fibers are strong and have a high modulus of elasticity, but their adhesiveness to rubber and their fatigue resistance are poor. On the other hand, PVA based fibers are strong and have a high modulus of elasticity, and their adhesiveness to rubber is good. In addition, their dimension changes little after vulcanization. For these reasons, PVA based fibers are widely used for rubber-reinforcing materials for hoses, belts, etc.
Increasing the seat area in automobiles is desired for comfortable drives. Contrary to this, however, the recent tendency is toward down-sized automobiles, for which the engine room and the tire wheel space is reduced. In many cases to meet this requirement, a brake hose is fitted to a narrower space than previously, at an extremely small bend angle having a radius of curvature of at most 30 mm, often preferably at most 20 mm, and is repeatedly expanded and compressed under severe conditions. In addition, to reduce the maintenance cost and treatment of automobiles, brake hoses are desired to have more high-level fatigue resistance. Similarly, reinforcing materials for tire cords, belts and others are also desired to have more high-level fatigue resistance.
In addressing the situation above, dipped cords have been proposed having increased strength and elasticity and having enhanced fatigue resistance, for example, in Japanese Patent Laid-Open No. 207338/1994, in which fibers of PVA having a high degree of polymerization are used. The PVA based fibers have an increased strength and an increased modulus of elasticity. To form them into dipped cords, cords of the PVA based fibers are dipped in an RFL liquid with their tension being lowered so that RFL can penetrate deep into the cords, and, after having been thus dipped, the cords are subjected to thermal fixation with their tension being increased so as to reduce twist irregularity. In this method, however, high-strength high-elasticity fibers of PVA of which the polymer chains are oriented to a high degree are used. Therefore, even though the dipping condition for the PVA based cords is specifically controlled in the manner as above, the fatigue resistance of the dipped cords cannot still be enhanced to a satisfactory degree. Their fatigue resistance is still low under the severe conditions described above.
On the other hand, Japanese Patent Laid-Open No. 189066/1995 discloses dipped cords of enhanced resistance to fatigue, for which high-elongation PVA based fibers are used. The PVA based fibers are prepared by suitably controlling the draw ratio and therefore have a high degree of elongation at break that falls between 6 and 12%. Japanese Patent Laid-Open No. 218221/1996 discloses using PVA based fibers for rubber reinforcing materials, etc. To enhance their fatigue resistance, abrasion resistance and dimensional stability, the PVA based fibers are treated for thermal shrinkage of from 1 to 15%, preferably from 2 to 10%, thereby making the orientation in their amorphous segments disordered. However, it is still difficult to satisfactorily enhance the fatigue resistance of dipped cords and fibers by merely increasing the degree of their elongation at break.